This invention relates to a process for preparing cellulose ethers.
Cellulose ethers are conventionally prepared by the reaction of alkali cellulose with diverse etherifying agents. The alkali cellulose employed is generally prepared from cellulose and an alkali metal hydroxide. The alkali metal hydroxide must be added to the cellulose as a concentrated aqueous solution in order to distribute the alkali metal hydroxide through the cellulose. Unfortunately, many etherifying agents react with water under the conditions of the etherification reaction to form undesirable by-products. Such side reactions cause the waste of substantial quantities of etherifying agent. Typically, as much as 70 percent of the etherifying agent used in the etherifying reaction goes to form such by-products. These by-products must then be removed from the product and recovered, thus adding additional expense to the process.
In British Pat. No. 546,647, it was attempted to improve the efficiency of cellulose etherification reactions by employing substantially anhydrous alkali cellulose as the starting material. The anhydrous alkali cellulose is prepared by mixing cellulose with an aqueous sodium hydroxide solution in the presence of an organic diluent and then distilling an azeotropic mixture of water and diluent under reduced pressure and moderate, i.e., 40.degree.-60.degree. C., temperatures. Unfortunately, however, said process adds significantly to the total time required for the etherification process and does not improve the efficiency of the etherification reaction as much as desired.
Similarly, in U.S. Pat. No. 4,341,892, it is taught to prepare alkali cellulose having a low water content by suspending cellulose in an organic solvent and contacting the suspension with an alkali metal hydroxide/alcohol mixture and an aqueous alkali metal hydroxide solution. Part of the water and all of the alcohol are subsequently distilled, forming a low water alkali cellulose. This process, however, requires substantial amounts of time and energy and the use of an extra organic reagent, all of which add substantially to the expense of the overall etherification process. Accordingly, it would be highly desirable to have a process whereby the etherification of cellulose is conducted with higher efficiency and which does not have the drawbacks associated with prior methods.